1. Field of the Invention
The invention relates to a method for detection of possible tissue injuries based on cavitational actions during the medical application of high sonic energy, which is generated extraneously by a sonic transducer and transmitted on to and into the patient's body via coupling means, the sonic field traversing a spatial section internal to the body and being focused on a target area or zone, to destroy a section present within the target area, for example concretion or a tissue section.
2. Description of the Prior Art
Upon exposing a human body to ultrasonic or shock waves for the purpose of destroying concretions, there is a risk of damaging the tissue for a variety of reasons. These include thermal injuries on the one hand, which may be prevented comparatively easily by adjusting the sonic dosage, and on the other hand damage caused by cavitational actions which are commonly difficult to predict and avert, since it is not always known precisely whether and when cavitational actions should be expected within the area exposed to sonic action.
Furthermore, it is very difficult when generating powerful sonic pressures, such as are required for example to break up stone concretions within a kidney, the gall-bladder or the bladder, to generate purely positive sonic pressures only. As a matter of fact, substantial negative pressures frequently also arise, leading to cavitational actions, in which the body fluid is practically disintegrated and gas bubbles are generated which may cause tissue cells to explode.
There are other fields moreover for the medical application of high-power ultrasonics, that is to say in the case of cancer treatment for example, in which cavitational actions are required however to destroy tissue cells and "tissue damage" is thus engendered deliberately by cavitational actions. In such cases too the detection of cavitational actions is of importance particularly since the destruction of unaffected tissues should be prevented and since it is of importance for the physician in charge to know whether and when the cavitational actions occur in the patient's body. SUMMARY OF THE INVENTION
It is an object of the invention to provide a method which allows early detection of cavitational actions which might give rise to injury and thereby creates a situation in which the physician in charge may intervene opportunely and by appropriate measures prevent grave injury to the patient.
This object is achieved in accordance with the invention in that for example during the exposure of the body to high sonic energy, or during brief intervals between separate exposures to sound, the target area is acted upon by an acoustic test signal, a reception signal is formed from at least one reflection of the test signal said reception signal is checked by comparison to the test signal to discover whether a marked changed in impedance (referred to herein as an "impedance jump") is present causing a weak sonic reflection, and a control signal is generated upon detecting an impedance jump of this nature.
According to the invention, use is thus made of the fact that a sound-absorbent reflector rotates the phase of a sonic signal incident from a less absorbent medium through 180.degree., so that positive pressure surges are reflected as negative pressure surges, and vice versa. It is thus possible to deduce from a change of polarity of the sonic signal, that the signal must have struck a sound-absorbent reflector and that an impedance jump will be present thereat, for example between tissue or fluid and gas.
Knowing the polarity of the test signal, it is now possible to detect without ambiguity by comparison of this polarity with the polarity of the reflected signal, whether an unattenuated reflection is present in view of identical polarities or whether an attenuated sound reflection is present in view of dissimilar polarities. Since reflectors which are sound-absorbent compared to water are not normally found in the body, excepting gases in the lungs or intestinal tract, the detection -of such attenuated reflections is a comparatively certain indication of the occurrence of cavitational actions and a motivation for generating the control signal which may be evaluated in a variety of ways, for example as a warning signal.
The nature of the reflection may be detected in the simplest case by oscilligraphic display and observation of the signals in question. Since it is preferable however to have an automatic detection and analysis, inclusive of generating a control or alarm signal, the reception signal may for example be observed within a time slot, by storing a half-cycle of the reception signal within a memory or hold circuit and comparing the same to the co-ordinated half-cycle of the test signal in respect of polarity, so that the control signal may be generated upon detecting different polarities. In this connection, if the initial half-cycle of the test and reception signals are analysed and the first half-cycle of the test signal always has positive polarity, the hold circuit should consequently be able to react only to negative reception signals in principle, without the need to scan for the specified polarity of the test signal and for an immediate comparison in each case between the polarities of both half-waves.
Other aids may be applied as well for the purpose of signal analysis, instead of the aforesaid time slot. In this connection, it is possible to contemplate an amplitude gap as well as other data processing methods, say such as conversion, correlation, cepstrum and the like or a combination of such methods known per se.
To capture and evaluate an impedance jump caused by a cavitational action, it is also possible moreover to have recourse to the detailed structure of the reflection or reflections, such as the amplitude of the individual reflected half-waves and/or amplitude sequence of several such half-waves and to plot these for evaluation in correlation or comparison with the amplitude and/or amplitude sequence of the test signal.
The possibility exists furthermore of exploiting not only the initial reflection or echo of the test signal as a reception signal, but also one or more multiple echoes between the target area or target object and the test pulse transmitter or an appropriate auxiliary reflector, the graph shapes of the echoes in question then being considered and interpreted appropriately for evaluation purposes.
Finally, it is not absolutely necessary to perform the momentary evaluating actions under application of actual electronic circuitry, since the actions and measures in question may also be formulated as an algorithm of a computer program. In this connection, it is also possible to encompass and evaluate an enlarged zone by scanning a longer period of the reception signal and seeking a signal which corresponds to a signal occurring in the case of an attenuated echo, in the computer by establishing the cross-correlative function between the actual reception signal and a reference signal.
A logical application of the inventive method may be considered notwithstanding the wide range of possibilities listed for detection and interpretation of the impedance jumps of interest for this purpose, in particular in conjunction with an existing system for location and disintegration of stone concretions. Known systems of this nature are illustrated and described, for example in the German OS text Nos. 2722252, 3122056 and 3220751. In essence, they comprise a high-power sonic transmitter and a locator system comprising at least one monitor whereby the focus of the sonic transducer may be aligned under observation on the concretion lying in the target area.
As already stated, this sonic transmitter may be utilised to generate the test signals on the one hand, and on the other hand also for reception of the echo or reflection signals.
On the other hand, the possibility evidently also exists that a separate transmitter may be provided for the test pulses and a separate receiver may be provided for reception of the echo signals, in which connection the transmitter and receiver should then be coupled mechanically with the high-power sonic transducer and the location system so that a geometrically unequivocal association and correlation between the target zones of both the transmitting and receiving systems are always assured.
It is possible moreover that a monitor of the location system which is available in any case may be utilised for the optical display of the warning signal derived from the control signal, in which connection a warning colouration, warning symbols or the like may for example appear on the image screen of the monitor. On the other hand, the warning signal may also be offered to the user as an audible or other perceptible signal.
Notwithstanding whether the high-power sonic transmitter is or is not co-opted to generate the test signals, it should be deactivated automatically at the first or repeated appearance of warning signals, or be operable under reduced power at least, to prevent tissue damage caused by intervening cavitational actions and to provide the user with the possibility of initial examination of the situation and of reaching a decision on whether the high-power sonic transducer may be placed in operation again or whether a renewed location of the concretion and a more precise alignment of the transducer focus on the concretion are needed. The same evidently and logically also applies in the case of locating tissue sections which should be destroyed and of the alignment of the focus on such tissue section.